Nature, origin and significance of the Fomopéa Pan-African high-K calc-alkaline plutonic complex in the Central African fold belt (Cameroon)

Journal of African Earth Sciences 57 (2010) 79–95

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Journal of African Earth Sciences

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Nature, origin and significance of the Fomopéa Pan-African high-K calc-alkalineplutonic complex in the Central African fold belt (Cameroon)

Maurice Kwékam a,*, Jean-Paul Liégeois b, Emmanuel Njonfang c, Pascal Affaton d, Gerald Hartmann e,Félix Tchoua f

a Département des Sciences de la Terre, Faculté des Sciences, Université de Dschang, B.P. 67 Dschang, Cameroonb Département de Géologie, Musée Royal de l’Afrique Centrale, B-3080 Tervuren, Belgiumc Laboratoire de Géologie, Ecole Normale Supérieure, Université de Yaoundé I, B.P. 47, Yaoundé, Cameroond CEREGE, Aix-Marseille Université, CNRS, B.P. 80, Europôle méditerranéen de l’Arbois, 13545 Aix-en-Provence cedex 04, Francee Geowissenschaftliches Zentrum Göttingen, GZG, Abt. Geochemie, Universität Göttingen, Goldschmidtstraße 1, 37077 Göttingen, Germanyf Laboratoire de Géodynamique interne, Faculté des Sciences, Université de Yaoundé I, B.P. 812 Yaoundé, Cameroon

a r t i c l e i n f o

Article history:Received 2 December 2008Received in revised form 11 July 2009Accepted 23 July 2009Available online 3 August 2009

Keywords:High-K calc-alkaline granitoidsPan-AfricanPost-collisionLinear lithospheric delaminationFomopéa

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* Corresponding author. Tel.: +237 77516684.E-mail addresses: [email protected] (M. Kw

africamuseum.be (J.-P. Liégeois), [email protected] (P.

a b s t r a c t

The Fomopéa plutonic complex in West Cameroon comprises three petrographical units: a biotite–horn-blende granitoid (BHG) including some diorite, a biotite monzogranite (BmG) and an edenite syenogra-nite (EsG). Amphibolites occur within each unit. The massif was emplaced into a Pan-Africanamphibolite-facies metamorphic basement. All rocks display igneous textures and are chemically calc-alkaline, the BHG being metaluminous (ASI < 1) the BmG (ASI = 0.98–1.06) and EsG (ASI = 0.94–1.1) beingmetaluminous to weakly peraluminous. U–Pb dates on zircon give a Pan-African age of 620 ± 3 Ma for adiorite and 613 ± 2 Ma for a quartz–monzodiorite, both belonging to the BHG. Sr–Nd isotopic data indi-cate the mixing between a juvenile source, probably the mantle (nearest Fomopéa pole: eNd(620 Ma) = +4and 87Sr/86Sr(620Ma) = 0.703) and a Palaeoproterozoic to Archaean lower continental crust (nearestFomopéa pole: eNd(620Ma) = �16 and 87Sr/86Sr(620Ma) = 0.709; Nd TDM = 2.9 Ga) through a contaminationprocess or through a bulk mixing event at the base of the crust. Evidence for both processes is providedby the coexistence of mafic enclaves and gneissic xenoliths within the granitoids. We propose a modelwhereby linear lithospheric delamination occurred along the Central Cameroon shear zone (CCSZ) inresponse to post-collisional transpression. This delamination event induced the partial melting of themantle and old lower crust, and facilitated the ascent of the magmas. The emplacement of numerouspost-collisional Neoproterozoic plutons along the CCSZ during the Pan-African orogeny indicates that thisprocess was of paramount importance. The continental signature and geophysical data from the areaindicate that the CCSZ corresponds to the northern lithospheric boundary of the Archaean Congo craton,and that the events recorded here correspond to the metacratonic evolution of the northern boundary ofthe Congo craton.

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1. Introduction

The Neoproterozoic Pan-African belt of Cameroon is a part ofthe Central Africa Pan-African belt. It connects to the west withthe coeval Trans-Saharan orogenic belt; to the south it is in contactwith the Archaean Congo craton, and to the north it borders theSaharan metacraton (Abdelsalam et al., 2002). This belt is con-nected with the northeastern Borborema Province of Brazil, whichis part of the Neoproterozoic Brasiliano belt (Almeida et al., 1981;Brito et al., 2002; Lerouge et al., 2006).

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ékam), jean-paul.liegeois@Affaton).

Early work on the Central Africa Pan-African fold belt focusedmainly on the tectonic evolution of the belt, and the general as-pects of magmatic activity associated with it (e.g. Tubosum,1983; Nédélec et al., 1986; Toteu et al., 1987, 1990; Barbey et al.,1990; Affaton et al., 1991; Ekwueme et al., 1991; Ngako et al.,1991, 1992, 2003; Penaye et al., 1993; Ferré et al., 1998, 2002).After a structural synthesis of the Eastern Nigeria and NorthernCameroon belts, Ngako (1999) suggested a continent–continentcollision for the area comparable to the Himalaya collision. Thismodel supposes the existence of one Archaean block in the Saharawhich moved from NNE to SSW about 630 Ma ago forming theCentral African fold belt along the Archaean Congo craton. Thisblock is now identified as the Saharan metacraton (Abdelsalamet al., 2002).

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80 M. Kwékam et al. / Journal of African Earth Sciences 57 (2010) 79–95

The Pan-African belt of Cameroon includes three structural do-mains separated by major shear zones (Zones 1, 2, 3 in Fig. 1): (1)the southern domain comprises Pan-African metasedimentaryunits, whose protoliths were deposited in a passive margin envi-ronment at the northern edge of the Congo craton; (2) the centraldomain includes Palaeoproterozoic metasediments and orthog-neiss intruded by widespread syn- to late-tectonic granitoidsmainly transitional in composition and of crustal origin (Sobaet al., 1991; Tchouankoué, 1992; Ganwa, 1998; Tchakounté,1999; Toteu et al., 2001) and having high-K calc-alkaline affinities(Kwékam, 1993; Talla, 1995; Nguiessi-Tchankam et al., 1997;

Fig. 1. Geological map of Cameroon, showing the major lithotectonic domains; ASZ,Cameroon shear zone; NT, Ntem complex; DS, Dja group; NS, Nyong group; 1 = southerdomain.

Njonfang, 1998; Tagne-Kamga, 2003; Nzolang et al., 2003); (3)the Northern domain consists of the Poli series (Ngako, 1986; Njel,1986; Toteu, 1990) that possesses the most oceanic character ofthe various Neoproterozoic basins present in Cameroon (Toteuet al., 2006a), Pan-African granitoids of mainly calc-alkaline com-position emplaced between 660 and 580 Ma (Toteu et al., 1987,2001), late alkaline granitoids and numerous basins made up ofunmetamorphosed sedimentary and volcanic rocks.

Here we present the Fomopéa Pan-African magmatic complex,emplaced along the Central Cameroon Shear Zone (CCSZ) that sep-arates the central from the northern domain (Fig. 1) as a large

Adamawa shear zone; SF, Sanaga fault; TBF, Tcholliré-Banyo fault; CCSZ, Centraln; 2 = central domain (Adamawa–Yadé domain of Toteu et al., 2004); 3 = northern

Fig. 2. Extract of West-Cameroon geological map from Dumort (1968) showing the locality of the Fomopéa plutonic complex and its neighbouring plutons (e.g. Batié,Ngondo, Bangangté, etc.).

M. Kwékam et al. / Journal of African Earth Sciences 57 (2010) 79–95 81

number of similar Pan-African granitoids (Fig. 2). Cameroon Pan-African granitoids are mostly post-collisional and considered tohave emplaced at around 580 Ma (Kwékam, 1993; Talla, 1995;Nguiessi-Tchankam et al., 1997) from a source of mixed origin(Tagne-Kamga, 2003) or made up of recycled old crustal material(Nzolang et al., 2003; Penaye et al., 2004; Nzolang, 2005; Kwékam,2005). Based on field, petrographical, major, trace and Rb–Sr, Sm–Nd, U–Pb isotope data, this work attempts to define the nature, ori-gin and age of the Fomopéa granitoids for constraining their geody-

namic setting in the Pan-African belt of Cameroon, whichknowledge is essential for deciphering West African geology, as Fo-mopéa is located between the West African craton, the Congo cra-ton and the Saharan metacraton.

2. Structural setting

The Neoproterozoic Pan-African belt in West Cameroon com-prises granito-gneissic units intruded by mafic to felsic plutons


(including the Fomopéa complex) and overlain by Cenozoic volca-nic complexes.

Four Pan-African regional tectonic phases have been recognizedin Cameroon (Toteu et al., 2004). The D1 phase created a flat-lyingfoliation and would correspond to early Pan-African nappe tecton-ics, verging east. The D2 deformation formed a foliation with steepaxial planes and is associated with tight and upright folds and withsyn-migmatitic conjugate shear zones; the D2 deformation is con-sidered to be the result of an E–W regional shortening directioninducing a NE–SW transcurrent movement. Both D1 and D2 phasesare associated with a N110–140�E stretching lineation. The D3

phase is related to sinistral movements along the N–S to NE–SWshear zones. The final D4 phase is characterized by N80–110�E dex-tral and N160–180�E sinistral shear zones. The D1–D2 phase arecontinued to the 635–615 Ma age range, the D3 to the 600–580 Ma age range and the D4, poorly dated, is older than 545 Ma(Toteu et al., 2004, and references therein). These four phases, asso-ciated to collisional and post-collisional evolution in the Cameroondomain, have been more recently identified as corresponding tothree successive tectonic events (Ngako et al., 2008): (1) crustalthickening (ca 630–620 Ma) including the thrusting (D1) and short-ening (D2) of Toteu et al. (2004); (2) the left lateral wrench move-ments (613–585 Ma) among which the Sanaga Shear zone,corresponding to D3 of Toteu et al. (2004); (3) the right lateralwrench movements (ca. 585–540 Ma) mainly marked by the Cen-tral Cameroon Shear Zone (CCSZ) and corresponding to D4 of Toteuet al. (2004). The CCSZ is a 70�E striking major crustal structureextending from Sudan into NE Brazil. It defines a fan-geometry incentral Cameroon, due to its interaction with N40�E directed shearzone system (Ngako et al., 2003). The CCSZ seems to have been ini-tiated earlier than the D4 phase, during the left lateral wrenchmovements, as indicated by sinistral shear sense indicators withthe same movement direction (Ngako et al., 2003; Njonfanget al., 2006, 2008) compatible with successive shearing events,within the dextral shear zones. The tectonic evolution of the Pan-African belt in central and southern Cameroon is characterizedby transpression, because of coeval shear and thrust kinematicinteractions. The resulting structures include the N70�E sinistralshear zones of central Cameroon (CCSZ and Sanaga fault) and thegranulitic and migmatitic thrust sheets overriding the Congo cra-ton (Ngako et al., 2003).

In the Fomopéa area, the granito-gneiss country rocks are af-fected mainly by the shortening phase of crustal thickening. Theschistosity and foliation planes are mainly oriented N110�E andare associated with folds and a stretching mineral lineation ofthe same orientation. They are often affected by N70�E ductileshear zones and are cut by N70�E migmatitic leucosomes corre-sponding to the second tectonic event. The magmatic fabrics(flowage) observed in the Fomopéa plutonic complex are orientedN–S to NNE–SSW (N0�–N30�E), parallel to the flow deformationfound in the anatectic granite (e.g. Dschang granite) outcroppingin the country-rocks. They dip towards E to ESE, in the oppositedirection to that of the anatectic granite (towards NW), and arethus not parallel to the foliation of the gneissic country-rocks.However, the Fomopéa complex, like other neighbouring massifs,is elongated parallel to the regional foliation. This suggests thatthe geometry of the Fomopéa complex is linked to a late crustalthickening event and could be the result of the same regionalstress in response to the contrasted rheological behaviour of thecountry rocks relatively to the moving Fomopéa magmas. Thenorthern and northeastern edges of the massif are overlain byCenozoic trachytic and basaltic lavas. Recent sedimentary rockspartially overlay its south-western edge, the remaining beingblurred by the presence of post-magmatic dextral NE–SW faultsassociated with mylonite (Kwékam, 1993), probably related tothe last tectonic event.

3. Field geology of the Fomopéa plutonic complex

The Fomopéa complex is made up of three main groups of plu-tonic rocks (Fig. 3): biotite–hornblende granitoids (BHG, includingsome dioritoids), a biotite monzogranite (BmG) and an edenite sye-nogranite (EsG). Small lens-shaped bodies, bands or masses withcumulus texture of amphibolitic rocks are often found in each group.

The BHG is located around Fomopéa village. The main rock typesare a porphyritic monzogranite usually grey in colour, and a darkgreen quartz–monzodiorite; subordinate quartz–monzonite andequigranular granodiorite are also present. Spindle-shaped en-claves of diorite and angular xenoliths of gneissic rocks are fre-quently observed. These enclaves are commonly elongatedparallel to the magmatic foliation, defined by the alignment ofamphibole, biotite, platy feldspar and schlierens. Schuppen struc-tures present in a granitic ‘‘proto-dyke” indicate a dextral strike-slipductile fault oriented N100�E.

The BmG forms a large homogeneous body to the NE in the Ba-loum area but is also crosscutting dykes intruding the BHG. Micro-dioritic enclaves are common, particularly along the contact withthe BHG.

The EsG forms a body located to the SE, around the Fontsa-Toulaand Fotoufem villages. It contains small enclaves of diorite (<5 cmin diameter). The contact with the south-western border of theBHG is progressive and often blurred by amphibolite panelswhereas the contact with the gneissic country rocks is sharp. Themagmatic foliation is marked by the alignment of amphibole andrare schlierens of amphibole.

The magmatic foliation in the different petrographic groups issub-parallel to the massif elongation (NNE–SSW dipping ESE)and the regional foliation, giving it a sheet-like structure and sug-gesting syn-kinematic emplacement. This magmatic foliation is ob-served throughout the BHG unit (N0–30�E) with dips varying from50�ESE in the margin to 85�ESE in the core, and rather lower to themargins of the BmG (N0–10�E, N50–75� ESE or E) and EsG (N10–15�E, N30–50� ESE). However, features of a syn- to late-kinematicpluton relative to the late crustal thickening event are suggested bythe alignments of the minerals, and enclaves of diorite and gneissiccountry rocks in the presence of schuppen structures. Indeed, theFomopéa pluton can then be considered as an early post-collisionalPan-African pluton, older than the abundant syntectonic graniticplutons (Toteu et al., 2006a) of the second to third tectonic events.The whole massif is fringed on its NE–SW edge by amphibolitebands that are mainly oriented N40�E with a dip of 60� to the SE,and on its SE edge by N50�E dextral mylonitic band, and a few latedykes of microgabbro crosscut the Fomopéa pluton.

4. Petrography

The biotite–hornblende granitoids (BHG) include diorite,quartz–monzodiorite, quartz–monzonite, granodiorite and monz-ogranite. The rocks are medium- to coarse-grained, with biotite,hornblende, plagioclase, K-feldspar and quartz as the major min-eral phases and magnetite, zircon, apatite, titanite and allanite ascommon accessory phases. The rocks display well-defined mag-matic textures. Plagioclase is often zoned and has andesine compo-sition (An50–35). Green, brownish to reddish biotite (XMg = 0.45–55)is common. Hornblende, the main amphibole, occurs both as euhe-dral prisms and as anhedral crystals often surrounding euhedralflakes of biotite. Its chemical composition vary from tschermakitichornblende (XMg = 0.50–0.60) in diorite to magnesio-hornblende(XMg = 0.54–0.56) in the BHG. Titanite, the most abundant acces-sory mineral, may reach more than 4% volume in some samples.Secondary minerals, chlorite, actinolite and epidote occur as sub-solidus alteration products of biotite, hornblende and plagioclase.

Fig. 3. Geological map of the Fomopéa plutonic complex.


The biotite–monzogranite (BmG) is fine- to medium-grainedand heterogranular. The main mineral phases are brownish to red-dish biotite (XMg = 0.45–0.50), zoned plagioclase (An25–30), K-feld-spar (microcline) and quartz whereas the accessory mineralsremain the same as in the BHG except allanite which was notfound. Secondary minerals are sericite and chlorite, resulting fromthe alteration of feldspar and biotite, respectively, while muscoviteoccurs as a hydrothermal mineral. Late magmatic to subsolidusdeformation imprints such as undulose extinction in quartz grains,minor sub-grain development along quartz–quartz boundaries,mechanical twinning in feldspars and herringbone cleavage(kink-band) in biotite are common in this monzogranite.

The edenite–syenogranite (EsG) is leucocratic, medium-grainedand heterogranular. It differs from the BmG mainly by the absenceof biotite and the occurrence of amphibole. As a whole, it containslarger quartz crystals, K-feldspar, plagioclase (An5–25, dominantlyalbite) and green sub-euhedral to euhedral hornblende(XMg = 0.50–0.70). Accessory minerals include titanite, apatite andFe–Ti oxides. As in the BmG, undulose extinction in quartz grainsand mechanical twinning in feldspars are observed.

The chemical compositions of biotite from the Fomopéa rocksplot in the calc-alkaline field of Nachit et al. (1985) in Kwékam(2005). The amphibole geothermobarometer of Johnson and Ruth-erford (1989) calculated for magmatic hornblende gives a temper-ature range of 710–760 �C with pressure varying from 2.2 to

4.8 kbar. Higher grade conditions are obtained in the diorite andlower ones in the edenite syenogranite.

5. U–Pb on zircon and Rb–Sr on whole-rock geochronology andSr–Nd isotopes

5.1. Analytical techniques

U–Pb analytical data were obtained at the Isotopic Geochemis-try Laboratory (IGL), University of Kansas, following the proceduresdescribed by Toteu et al. (1994). All zircons analyzed were air-abraded for 2–4 h to remove as much of the altered outermost partas possible (Krogh, 1982). All U–Pb analyses were performed onsingle grains; all our samples were spiked with 205Pb–235U tracersolution before dissolution. Results are given in Table 1.

Sr isotopes on whole rocks were measured at the Royal Museumfor Central Africa, Tervuren (Belgium). After dissolution in an HF/HClO4 mixture, Sr was separated on Dowex 50/150 ion exchangeresin. Total chemical blanks range between 10 and 15 ng Sr. Mea-surements were carried out on a GV Sector 54 TIMS. An error of 4%on Rb/Sr ratios is estimated on standard and sample replicates. Theexternal reproducibility on NBS987 standard solution was0.710255 ± 0.000015 during the course of this study. Sr composi-tions were corrected to a value of 0.710250 for NBS987 and frac-

Table 1U-Pb zircon data obtained on different fractions extracted from the dioritic sample K204 and quartz monzodioritic sample K104.

Sample Isoplot data Calculated ages

Fraction Size(mg)

U(ppm)

Pb(ppm)

Pb206/Pb204(obs.)

Pb207/U235

±2r(pct)

Pb206/U238

±2r(pct)

Correl.Coeff.(rho)

Pb207/Pb206

±2r(pct)

Pb206/U238Age

±2r(Ma)

Pb207/U235Age

±2r(Ma)

Pb207/Pb206Age

±2r(Ma)

DioriteK-204 Zr-1 0.007 678 71 2067 0.8439 0.50 0.10123 0.48 0.956 0.06046 0.15 621.6 3.0 621.3 3.1 620.1 3.2K-204 Zr-2 0.006 826 73 2676 0.6814 0.50 0.08226 0.48 0.969 0.06008 0.12 509.6 2.5 527.6 2.6 606.5 2.7K-204 Zr-3 0.004 147 15 882 0.8945 1.10 0.10614 1.05 0.963 0.06112 0.30 650.3 6.8 648.8 7.1 643.4 6.3

Quartz–monzodioriteK-104 Zr-1 0.007 508 52 1592 0.8250 0.59 0.09935 0.56 0.962 0.06023 0.16 610.6 3.4 610.8 3.6 611.8 3.5K-104 Zr-2 0.003 1095 112 1651 0.8305 0.53 0.09998 0.51 0.972 0.06024 0.12 614.3 3.2 613.9 3.2 612.3 2.7K-104 Zr-4 0.006 975 102 348 0.7439 0.53 0.08940 0.47 0.907 0.06035 0.22 552.0 2.6 564.7 3.0 616.0 4.8K-104 Zr-3 0.006 306 34 1375 0.8451 0.57 0.10112 0.56 0.974 0.06061 0.13 621.0 3.5 621.9 3.6 625.6 2.8K-104 Zr-5 0.010 377 42 1450 0.8482 0.51 0.10137 0.49 0.966 0.06068 0.13 622.5 3.1 623.7 3.2 628.0 2.8

Table 2Rb–Sr isotopic whole-rock data.

Rb (ppm) Sr (ppm) 87Rb/86Sr 2r 87Sr/86Sr 2r (87Sr/86Sr)i

620 Ma 700 Ma

Diorite K19 26.8 1037 0.0748 0.003 0.703953 0.000011 0.70329 0.70321BHG Fk12 144 539 0.7734 0.031 0.711422 0.000010 0.70458 0.70369BHG K34 163 427 1.1054 0.044 0.714176 0.000009 0.70440 0.70313BHG K35 139 468 0.8599 0.034 0.712336 0.000010 0.70473 0.70375BHG K70 145 488 0.8602 0.034 0.712058 0.000007 0.70445 0.70346BHG K75 121 598 0.5857 0.023 0.709946 0.000010 0.70476 0.70409BHG K56 170 685 0.7184 0.029 0.711223 0.000011 0.70487 0.70405BmG K61 242 211 3.3279 0.133 0.735137 0.000009 0.70571 0.70192BmG K68 349 175 5.7984 0.232 0.755678 0.000010 0.70440 0.69775BmG K72 271 383 2.0513 0.082 0.726040 0.000010 0.70790 0.70554EsG K52 175 717 0.7068 0.028 0.714603 0.000010 0.70835 0.70754EsG Ft3 177 745 0.6880 0.028 0.714558 0.000010 0.70847 0.70768EsG K51 129 285 1.3115 0.052 0.720339 0.000009 0.70847 0.70724Amphibolites AM1 60.5 746 0.2347 0.0094 0.706210 0.000008 0.70414 0.70386Amphibolites AM2 27.6 355 0.2246 0.0090 0.706710 0.000007 0.70472 0.70446Amphibolites K71 52.6 498 0.3059 0.0122 0.707609 0.000010 0.70490 0.70455


tionation corrected to 86Sr/88Sr = 0.1194. Ages were calculated fol-lowing Ludwig (2003). Results are given in Table 2.

Nd isotopes on whole rocks were measured at the Geowissens-chaftliches Zentrum Göttingen-Isotopengeologie. Samples werespiked with a mixed 150Nd/149Nd Sm spike prior to dissolution.The separation was performed in two steps: cation exchange col-umns with HCl chemistry preceded a separation of Sm and Nd onTeflon columns coated with HDPE. The analyses were performedon a Finnigan MAT 262 TIMS operating in static mode. The totalanalytical error on the Sm and Nd abundances is <1%. The externalreproducibility on a La Jolla standard solution was0.511837 ± 0.000015. Nd compositions were corrected to a valueof 0.511858 for the La Jolla standard and fractionation correctedto 146Nd/144Nd = 0.7219. Single stage Nd model ages (TDM1) werecalculated according to the model of Michard et al. (1985) and Nel-son and DePaolo (1985). Two-stage Nd model ages (TDM2) werealso calculated assuming a mean crustal 147Sm/144Nd ratio of0.12 for the source prior to the intrusion age, using the evolvingmantle curve of Nelson and DePaolo (1985). Results are given inTable 3.

5.2. Age results

Three zircons from one diorite (K204) and five zircons from onequartz–monzodiorite (K104), both from the BHG, were analyzed.The results are presented in Table 1 and Fig. 4. Two diorite zirconsare concordant and one is discordant towards 0 Ma. The two con-

cordant zircons give, respectively, 648 ± 5 Ma and 621 ± 3 Ma. Thissuggests an intrusion age for the diorite at 621 ± 3 Ma and aninherited zircon at 648 ± 5 Ma. Four zircons from the quartz–mon-zodiorite are concordant and one is discordant towards 0 Ma. Twozircons are concordant at 623 ± 2 Ma and the two others at613 ± 2 Ma. This suggests an intrusion age for the quartz–monzodi-orite at 613 ± 2 Ma with inherited zircons at 623 ± 2, possibly com-ing from the diorite: combining the magmatic zircon from thediorite with those zircons gives an age of 622 ± 4 Ma with anacceptable MSWD of 1.3. This suggests that the Fomopéa complexwas emplaced within about 10 m.y. These U–Pb results on zirconindicate that the Fomopéa complex was emplaced at the end ofthe crustal thickening and represents an early generation ofhigh-K calc-alkaline plutons emplaced in Cameroon, being olderthan most of the Pan-African plutons that intruded at ca. 580 Ma,during the second tectonic event. It is contemporaneous to the on-set of the Yaoundé nappe tectonics towards the south (616–610 Ma; Toteu et al., 2006b).

Sixteen whole-rock Sr isotopes analyses were carried out andthe results are presented in Table 2 and Fig. 5. The six samples fromthe biotite–hornblende granitoid define an isochron with an age of572 ± 48 Ma (Sr initial ratio = 0.705520 ± 0.00053, MSWD = 1.15);the diorite is strongly below. The three samples from the biotite–monzogranite and the three from the edenite–syenogranite deter-mine an isochron of 561 ± 34 Ma (Sr initialratio = 0.70926 ± 0.00095, MSWD = 3.9). These ages are affectedby large errors and partly based on mixed groups, which weaken

Table 3Sm–Nd isotopic whole-rock data.

Sm(ppm)

Nd(ppm)

147Sm/144Nd 2r 143Nd/144Nd 2r eNd0143Nd/144Nd(620 Ma)

eNd620 TCHUR

(Ma)TDM1

(Ma)TDM2

(Ma)

DioriteK19 5.736 26.36 0.1316 0.0026 0.512563 0.000003 �1.5 0.512028 +4.04 176 923 898

BHGFk12 6.17 25.3 0.1475 0.0029 0.512320 0.000004 �6.2 0.511721 �2.31 985 1613 1657K34 6.75 30.5 0.1338 0.0027 0.512316 0.000006 �6.2 0.511776 �1.22 772 1364 1376K35 8.00 31.4 0.1541 0.0031 0.512328 0.000004 �6.0 0.511702 �2.67 1108 1751 1821K70 3.85 22.6 0.1030 0.0021 0.512291 0.000005 �6.8 0.511872 +0.66 565 1049 1038K75 5.83 29.5 0.1195 0.0024 0.512325 0.000004 �6.1 0.511839 +0.01 619 1167 1162K56 5.85 33.9 0.1044 0.0021 0.512253 0.000004 �7.5 0.511829 �0.19 636 1110 1103

BmGK61 4.28 27.3 0.0948 0.0019 0.511997 0.000008 �12.5 0.511612 �4.44 959 1332 1339K68 3.73 21.5 0.1049 0.0021 0.512082 0.000004 �10.8 0.511656 �3.58 923 1337 1344K72 4.38 32.8 0.0808 0.0016 0.511973 0.000003 �13.0 0.511645 �3.79 874 1226 1227EsGK52 2.88 11.9 0.1463 0.0029 0.511623 0.000004 �19.8 0.511028 �15.83 3051 2976 3286K51 2.40 11.8 0.1230 0.0025 0.512035 0.000007 �11.8 0.511535 �5.93 1246 1651 1689

AmphibolitesAM1 6.72 38.62 0.1052 0.0021 0.512068 0.000025 �11.1 0.511558 �3.60 950 1358 1368AM2 6.40 26.31 0.1471 0.0029 0.512499 0.000011 �2.7 0.511786 +1.60 428 1242 1244K71 5.56 24.0 0.1398 0.0028 0.512343 0.000013 �5.7 0.511775 �1.25 791 1705 1444

TDM1 = single stage Nd model ages; TDM2 = two-stage Nd model ages.


the information. However, some conclusions can be given: (1) theRb–Sr ages of the Fomopéa pluton are centred on the age of theright lateral wrench movements at 580 Ma that affected the com-plex and then also probably this isotopic system; (2) whatever thisreactivation, different initial ratios are recorded following the fa-cies considered with increasing Sr initial ratios with silica.

5.3. Nd–Sr isotopic characteristics

The 87Sr/86Sr initial ratios calculated at 620 Ma do not vary sig-nificantly within a given petrographical type: diorite (0.703),amphibolites (0.704), biotite–hornblende granitoid (0.704–0.705),biotite–monzogranite (0.704–0.708) and edenite–syenogranite(0.708–0.709). As for the Sr isotopes, the Nd isotopes are groupedwhen considering one petrographical type but show a large varia-tion when the whole Fomopéa complex is considered: the diorite(K19) displays the most juvenile characteristics with moderatelypositive eNd value (+4) and young TDM (TDM1 = 0.90/0.92 Ga andTDM2 = 0.89 Ga). The BHG (Fk12, K34, K35, K70, K75, and K56) haveeNd around 0 (between +0.9 and �2.4) with TDM1 between 1.04 and

Fig. 4. Concordia diagram for the Fomopéa plutonic complex showing (a) quartz–monzodiorite and (b) diorite U–Pb ages.

1.82 Ga and TDM2 between 1.03 and 1.82 Ga. The biotite monzogra-nite (K61, K68, and K72) have negative eNd (between �3.3 and�4.2) with TDM1 between 1.23 and 1.34 Ga and TDM2 between1.23 and 1.34 Ga. The edenite syenogranite has more crustal Ndsignature. The two samples analyzed (K51 and K52) have morenegative eNd (�5.7 and �15.6) with older model ages TDM1 of1.65 and 2.98 Ga and TDM2 1.69 and 3.29 Ga. Palaeoproterozoiccrust (2.1–1.66 Ga, U–Pb on zircon) is known in SW Cameroon(Penaye et al., 2004; Lerouge et al., 2006) and in Bafia region (Tcha-kounte et al., 2007) and Archaean crust (2.9 Ga, Rb–Sr isochron) inthe nearby Congo craton (Lassere and Soba, 1976; Shang et al.,2004, 2007).

6. Geochemistry

6.1. Analytical procedures

Major and some trace elements were measured at the ‘‘Institutfür Geowissenschaften, Christian-Albrechts-Universität zu Kiel,

Fig. 5. Isochron Rb–Sr diagrams: (a) plot of BHG and (b) plot of BmG and EsG.

Table 4Major (wt.%, recalculated to 100% on an anhydrous basis) and trace elements data for Fomopéa plutonic complex, n.m. and blank = not measured, b.d.l. = below detection limit.

Amphibolites Diorites Biotite–hornblende granitoids (BHG)

K22 AM2 AM1 K71 K19 SO1 FK6 FK5 K1 K35 K45 K54 K75 FK12 K4 K34 K40 K56 K70

Major elements (recalculated for 100%)SiO2 51.55 50.12 56.79 50.37 51.78 52.55 51.78 49.06 63.39 64.39 61.35 65.63 62.85 63.35 66.65 69.42 72.99 68.96 66.57TiO2 0.80 2.11 1.11 1.05 1.52 1.70 1.63 1.38 0.71 0.65 0.90 0.63 0.75 0.69 0.66 0.40 0.20 0.59 0.49Al2O3 14.07 14.22 16.46 11.85 19.03 16.71 20.13 18.71 14.93 15.56 16.61 15.14 15.30 16.40 14.97 14.38 13.91 15.16 15.17Fe2O3 10.08 13.97 7.94 11.51 8.64 10.45 8.77 11.49 5.72 5.36 6.50 4.71 6.01 5.89 4.88 2.79 1.51 3.06 4.25MnO 0.16 0.20 0.16 0.22 0.12 0.14 0.10 0.19 0.10 0.11 0.10 0.10 0.11 0.10 0.08 0.07 0.02 0.04 0.08MgO 9.06 5.37 4.29 9.40 3.51 4.45 3.23 4.98 2.25 2.50 2.43 1.51 2.68 2.52 1.95 1.49 0.56 0.97 1.90CaO 10.51 8.42 7.47 10.59 9.40 8.22 9.20 8.92 4.77 4.54 5.54 3.43 4.79 4.57 3.47 3.02 1.85 2.53 3.73Na2O 2.58 4.25 4.30 2.49 4.23 4.20 3.96 3.09 3.68 3.89 4.32 4.42 3.60 4.12 3.86 3.08 3.24 3.53 3.66K2O 1.18 1.05 1.12 1.11 1.08 1.26 0.87 1.69 3.31 3.52 1.75 4.09 3.21 2.73 3.73 4.99 5.40 4.92 3.72P2O5 0.11 0.33 0.44 0.24 n.m 0.32 0.33 0.50 0.22 n.m n.m n.m n.m 0.22 n.m n.m 0.04 0.23 n.mTotal 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100LOI 0.23 n.m n.m 0.50 0.70 n.m n.m n.m 0.51 0.45 0.49 0.28 0.6 0.35 0.5 0.31 0.29 0.53 0.41Na2O + K2O 3.76 5.29 5.41 3.60 5.31 5.47 4.83 4.78 6.99 7.41 6.07 8.51 6.82 6.86 7.59 8.07 8.64 8.46 7.38Mg/Mg + Fe 0.64 0.43 0.52 0.62 0.45 0.46 0.42 0.46 0.44 0.48 0.43 0.39 0.47 0.46 0.44 0.52 0.42 0.39 0.47A/CNK 0.57 0.61 0.75 0.48 0.75 0.72 0.83 0.81 0.82 0.84 0.87 0.84 0.84 0.91 0.90 0.90 0.96 0.96 0.90A/NK 2.55 1.75 1.99 2.24 2.34 2.02 2.70 2.71 1.55 1.53 1.84 1.29 1.63 1.68 1.44 1.37 1.24 1.36 1.51K2O/CaO + Na2O (molar) 0.09 0.08 0.09 0.09 0.08 0.09 0.06 0.14 0.34 0.36 0.16 0.43 0.34 0.27 0.42 0.69 0.83 0.66 0.43

Trace elements (ppm)Li 7.7 n.m n.m 35.9 35.4 39.6 58.4 39.5 14.4 26.8 33.5Sc 42.5 n.m n.m 80.2 66.2 55.6 35.9 54.6 46.1 40.5 35.3Co 38.9 n.m n.m 48.5 42.7 26.9 28 25.1 15.8 18.3 21.8Ni 153.2 n.m n.m 133 69.2 47.1 32.8 55 37.2 34.6 27.7Cu 115.7 n.m n.m 273 188 117 49 119 81.8 48.8 39.8Zn 124.7 n.m n.m 110 131 78.8 91.2 102 45.3 51.5 73.5Ga b.d.l. n.m n.m 31.1 29.2 26.4 27.9 27.3 21.5 23.5 25.3Rb 25 28 53 34 96 145 139 150 174 170 157Sr 328 355 498 1013 794 448 620 511 441 648 491Y 37.2 16.9 18.3 22.9 36.2 23.1 36.3 18.6 26.6 18.5 18.2Zr 174 173 110 153 85 178 181 173 99 181 173Nb 8.5 12.3 8.5 12.7 9.80 16.90 14.8 14.3 20.9 20.8 13.1Mo n.m n.m n.m 4.2 2.5 2.4 2.0 2.7 4.2 2.9 1.5Cs n.m n.m n.m 4.4 6.5 11.9 9.4 14.3 4.3 5.9 7.8Ba 208 557 265 466 671 635 867 443 889 1295 731La 22.2 38.8 18.2 23.4 29.7 39.5 44.4 29.3 35.9 45.8 45.8Ce 38.8 82.6 41.9 47.0 64.6 73.8 69.9 54.4 77.7 90 51.6Pr 5.3 10.0 5.3 6.0 8.4 8.2 8.2 6.3 9.0 9.9 6.7Nd 22.9 38.6 24.0 25.8 39.9 31.4 29.5 25.3 30.5 33.9 22.6Sm 6.1 6.7 5.6 7.9 8.9 8 5.8 6.2 6.8 5.9 3.9Eu 1.9 1.8 1.5 1.9 2.0 1.7 1.2 1.5 1.0 1.1 0.8Gd 6.6 5.9 5.4 5.1 8.1 5.7 5.6 5.3 5.3 4.7 4.3Tb 1.1 b.d.l. b.d.l. 0.8 1.4 0.7 0.7 0.6 0.9 0.8 0.5Dy 7.3 4.0 4.2 5.4 7.4 4.7 3.8 4.8 4.1 3.3 2.4Ho 1.7 0.8 0.9 1.2 1.5 1.0 0.8 0.8 1.0 0.7 0.5Er 4.4 2.2 2.3 3.0 3.6 2.0 2.6 2.6 3.0 1.9 1.4Tm 0.6 b.d.l. b.d.l. 0.6 0.7 0.4 0.3 0.4 0.6 0.4 0.3Yb 3.8 1.8 2.0 4.0 3.3 2.3 2.0 2.5 3.1 1.8 1.7Lu 0.6 0.3 0.3 0.5 0.6 0.5 0.3 0.4 0.5 0.4 0.3Hf 4.2 3.8 2.9 3.9 3.4 5.7 4.8 5.5 3.4 5.2 4.7Ta 0.4 0.7 0.4 1.0 0.8 1.3 0.8 1.3 1.9 2.2 0.8Tl 0.2 b.d.l. b.d.l. 0.7 0.8 0.9 0.6 1.4 1.2 1.0 0.7Pb 4.2 11.2 8.3 35.4 15.0 25.3 18.3 25.8 33.8 29.2 25.7Bi b.d.l. b.d.l. b.d.l. 0.5 0.5 1.1 0.4 0.6 0.6 0.5 0.2Th 2.0 4.6 5.5 4.1 2.9 18.2 11.2 18.1 45.6 38.6 30.4

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57(2010)

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U 0.9 1.0 2.8 1.3 1.2 7.4 3.6 8.0 7.5 3.9 7.3(La/Yb)N 3.94 14.21 6.06 3.95 6.00 11.69 15.07 7.81 7.82 16.92 18.56(La/Sm)N 2.25 3.56 2.01 1.83 2.05 3.05 4.70 2.93 3.28 4.83 7.34(Gd/Yb)N 1.31 2.77 2.25 1.22 1.80 1.47 2.20 1.48 1.44 1.43 1.77Eu/Eu� 0.92 0.87 0.82 0.91 0.73 0.78 0.64 0.82 0.54 0.65 0.60RREE 123.16 193.49 111.37 132.30 180.05 179.94 175.08 140.44 179.20 200.56 142.83

Biotite monzogranites (BMG) Edenite syenogranites (ESG)

K55 K13 K14 K61 K68 K58 K72 K74 MTO1 MTO2 K47 K50 K51 K52 FT3 FT4 FT50

Major elements (recalculated for 100%)SiO2 69.04 70.44 69.45 74.38 74.95 72.67 73.56 74.06 75.56 75.04 74.73 74.95 74.63 73.19 73.42 72.79 71.95TiO2 0.56 0.34 0.48 0.15 0.13 0.21 0.20 0.15 0.07 0.16 0.09 0.07 0.11 0.15 0.18 0.16 0.16Al2O3 14.59 14.77 14.85 13.63 13.68 14.78 14.19 13.55 13.96 14.16 14.49 13.79 13.70 9.94 14.49 14.41 14.81Fe2O3 3.00 2.58 2.91 1.21 0.94 1.47 1.48 1.17 0.62 1.14 0.98 0.91 0.94 1.68 1.48 1.59 1.56MnO 0.05 0.02 0.04 0.02 0.02 0.02 0.02 0.04 0.01 0.04 0.02 0.03 0.01 0.05 0.04 0.03 0.05MgO 1.22 0.61 0.78 0.19 0.14 0.10 0.22 0.18 0.00 0.40 0.03 0.03 0.38 0.39 0.13 0.35 0.40CaO 2.61 1.59 1.92 1.32 0.99 1.13 1.26 1.05 0.72 0.97 0.87 0.89 1.31 1.14 1.05 1.03 1.14Na2O 3.59 2.94 2.89 3.32 3.74 3.40 3.41 3.41 4.30 3.49 4.21 4.85 4.88 4.52 4.11 4.61 4.78K2O 4.64 6.15 6.30 5.33 5.07 5.81 5.13 5.88 4.74 5.60 4.32 4.18 3.80 4.82 5.06 4.96 5.11P2O5 n.m n.m n.m n.m n.m 0.05 n.m 0.03 0.01 0.03 0.01 n.m n.m n.m 0.03 0.06 0.05Total 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100LOI 0.53 0.53 0.37 0.42 0.31 0.28 0.39 0.3 n.m n.m 0.19 0.18 0.16 0.12 0.42 0.37 0.26Na2O + K2O 8.23 9.09 9.20 8.65 8.81 9.21 8.55 9.29 9.04 9.09 8.53 9.03 8.68 9.34 9.17 9.52 9.83Mg/Mg + Fe 0.45 0.32 0.35 0.24 0.23 0.12 0.23 0.24 0.00 0.41 0.06 0.06 0.44 0.32 0.15 0.31 0.34A/CNK 0.93 1.03 0.99 1.00 1.02 1.06 1.05 0.98 1.03 1.04 1.10 0.98 0.94 0.67 1.02 0.97 0.96A/NK 1.33 1.28 1.28 1.21 1.18 1.24 1.27 1.13 1.14 1.20 1.25 1.10 1.13 0.79 1.18 1.11 1.11K2O/CaO + Na2O (molar) 0.61 1.06 1.05 0.87 0.78 0.95 0.82 0.97 0.67 0.92 0.60 0.51 0.45 0.62 0.71 0.63 0.62

Trace elements (ppm)Li 17.0 49.5 36.7 n.m n.m 11.66 n.m n.mSc 23.9 26.5 13.8 10 2 48.4 3 3Co 7.4 6.8 4.9 7 n.m 13.4 2 2Ni 13.6 17.2 9.6 n.m n.m 38 n.m 110Cu 27.7 30.9 15.3 30 n.m 88.8 n.m n.mZn 39.8 43.7 64.9 60 n.m 42.5 n.m n.mGa 28.0 31.6 30.5 13 15 22.7 16 15Rb 256 366 310 144 109 178 178 162Sr 211 173 395 1134 282 698 724 762Y 12.1 11.3 7.4 32 12 12.9 18 13Zr 146 121 202 207 64 113 122 124Nb 14.5 15.2 9.0 12 6 10.8 8 7Mo 0.8 1.1 0.4 n.m n.m 1.0 n.m n.mCs 6.5 9.9 6.5 1.6 b.d.l. 5.4 4.6 2.8Ba 439 376 901 6721 344 1861 2007 2060La 55.8 41.5 69.3 73.2 13.3 21.6 34.3 17.5Ce 93.5 66.4 120 119 6.9 34.5 55.1 31Pr 9.1 6.8 11.1 12.7 3.1 3.9 4.6 2.9Nd 27.3 21.5 32.8 51.2 11.8 11.9 16 10.9Sm 4.3 3.7 4.4 9.7 2.4 2.9 3 2.1Eu 0.5 0.5 0.8 2.5 0.6 0.4 0.7 0.6Gd 3.5 2.3 3.0 7.8 1.9 3.2 2.3 1.7Tb 0.5 0.4 0.4 1.1 0.3 0.4 0.4 0.3Dy 2.1 1.9 1.4 5.7 1.9 1.8 2.4 1.7Ho 0.4 0.3 0.2 1.0 0.4 0.6 0.5 0.3Er 1.3 1.2 0.7 3.0 1.2 1.5 1.5 1.1Tm 0.2 0.2 0.1 0.5 0.2 0.3 0.2 0.2Yb 1.3 1.1 0.4 2.7 1.3 1.4 1.5 1.1

(continued on next page)

M.K

wékam

etal./Journal

ofA

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Sciences57

(2010)79–

9587

Tab

le4

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tite

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Germany” on fused discs by XRF technique. The 2r-error for majorelements is <1%, for trace elements <5%. All other trace elementswere determined by ICP-MS, with errors 2r < 20% for Nb and Ta,<10% for Cs, Cu, Hf, Li, Ni, Sc, Y, Pb, Th, and U, and �5% for Rb, Sr,and the REE. Major elements have been recalculated to 100% with-out the loss of ignition (LOI) before use in the geochemical study.Representative data from 37 rock samples of three main groupsfrom Fomopéa plutonic complex are given in Table 4.

6.2. Major elements

The SiO2 contents range from 49 to 52.6 wt.% in diorites to61.4% to 69.4 wt.% in the BHG and are higher and similar in theBmG (69.4–75.6 wt.%) and the EsG (71.9–75.0 wt.%). Harker dia-grams (Fig. 6) show a general linear decrease in Al2O3, TiO2,Fe2O3, CaO, and MgO with increasing SiO2 at least for SiO2 > 60%.The diorites and the BHG belong to a calc-alkalic series while theBmG and the EsG belong to an alkali-calcic series (MALI diagram,Frost et al., 2001; Fig 7). This difference can be seen in the Na2Ovs. SiO2 diagram (Fig. 6) where, with increasing SiO2, Na2O de-creases in the BHG group and increases in the BmG group withopposite trends for K2O (Fig. 8). In the total alkali–silica diagram(Na2O + K2O vs. SiO2), the three groups display a subalkaline affin-ity (Fig. 9a) whereas in the A/NK vs. A/CNK (Fig. 9b), BHG is meta-luminous (A/CNK < 1), both EsG and BmG are metaluminous orweakly peraluminous (A/CNK = 0.94–1.1 with 2 samples > 1) ex-cept sample K52 from the EsG which is peralkaline (A/CNK: 0.67;NK/A: 1.22) due to a very low Al2O3 content. There is a silica gap(52–62%) between the diorites and the BHG and BmG, partly filledby an amphibolite. This rises to the relation existing between thegranitoids. As we will see, the trace elements are similar all overthe silica range, pointing to a common origin and a likely linktrough a crystal fractionation process. Such a modelization wouldrequire more samples and is out of the scope of this study.

6.3. Trace and rare earth elements

The sliding normalization (each rock is normalized to the virtualrock from the reference series that has the same silica content)proposed by Liégeois et al. (1998) for distinguishing the potassicfrom the alkaline-peralkaline series confirms that no alkaline rocksare present in Fomopéa (Fig. 10). The Fomopéa rocks follow apotassic trend, parallel to the Y axis, implying a strong enrichmentin LILE compared to HFSE. This diagram show that the EsG is notvery rich in LILE if its silica content is taken into account, in agree-ment with the decrease in K2O with SiO2, due to the crystallizationof K-rich minerals such as K-feldspar.

As a whole, trace elements (Fig. 11) are distributed non-uni-formly in the three groups of rocks. The BHG group contains thehighest content of Nb (20.9 ppm, lowest value – 6 ppm – in EsG),Y (36.3 ppm, lowest value – 7.4 ppm – in BmG), Hf (5.72 ppm, low-est value – 2.1 ppm – in EsG) and Ni (133 ppm, lowest value –9.5 ppm – in BmG). The BmG group contains the highest contentsof Rb (366 ppm, lowest value – 34 ppm – in BHG), Th (59 ppm,lowest value – 1 ppm – in EsG) and U (14.8 ppm, lowest value –0.3 ppm – in EsG). The EsG group shows the highest content ofSr (1134 ppm, lowest value – 173 ppm – in BmG) and Zr(207 ppm, lowest value – 85 ppm – in BHG) and the highest andlowest content of Ba (6721 and 344 ppm); here, the samples withthe highest Ba (K47) and lowest Ba (K51) also have the highest andlowest Sr and Zr contents of the group (Table 4). With the excep-tion of those samples in the corresponding diagrams, rocks in theHarker diagrams behave differently from the group to anotherand sometimes within the same group with increasing SiO2

(Fig. 11). Rb is incompatible in the BHG, dispersed in the BmGand shows a convex down profile in the EsG with a maximum (on-

Fig. 6. Harker diagrams of selected major elements.


set of orthoclase crystallization) at 73.4 wt.% SiO2. Ba, although dis-persed, increases in the BHG and becomes compatible in the BmGand the EsG with steeper slope in the BmG. Sr decreases in the BHGin a dispersed way and remains compatible in the BmG and the EsGwith similar slopes as for Ba. Zr is also compatible in the BmG andEsG, with a steeper slope in the BmG, whereas Nb displays threetrends, each being incompatible, with steeper and parallel slopesin the BmG and EsG. The similar behaviour of Zr, Sr and Ba in thethree remaining samples of EsG is also attested by their constantratios (Sr/Zr = 5.93–6.18; Ba/Sr = 2.67–2.77; Ba/Zr = 16.45–16.61).Th is incompatible in BHG and EsG with more regular and steeperslope in BHG, and compatible in BmG. All these characteristics arecompatible with a calc-alkalic source and a crystal fractionationprocess, low values of elements such as Nb or Th in the most

Fig. 7. MALI diagram from Frost et al. (2001). Plot of Na2O + K2O � CaO vs. SiO2

showing the ranges for the alkalic, alkali calcic, calc-alkalic, and calcic rock series.Fomopéa granitoids are calc-alkalic (diorites and BHG) and alkali-calcic (BmG andEsG).

evolved ESG facies being explained by the crystallization of lateminerals uneasy to determine in the absence of the cumulates,generally formed in that case through filter-pressing (Hadj Kad-dour et al., 1998).

REE contents are generally low in the Fomopéa rocks (45–290 ppm). The BmG has higher concentrations (148–243 ppm) fol-lowed by the BHG (132–201 ppm); the EsG have <123 ppm totalREE with one sample (K47) at 290 ppm. The light rare earth ele-ments are more enriched in the BmG (LREE/HREE: 18.6–38.5) thanin the BHG (5.5–13.3) and the EsG (5.1–12.5). All rocks shows mod-erate negative europium anomalies excepted in amphibolites anddiorites (Fig. 12): the negative Eu anomaly (Eu/Eu*) is higher inthe BmG (0.40–0.64) than in the EsG (0.80–0.89, with one sampleat 0.35) and more variable in the BHG (0.60–0.91). The general pat-terns show LREE enrichment and almost flat HREE except in dior-ites. The Fomopéa REE patterns are similar to the Pan-African

Fig. 8. Classification of the Fomopéa plutonic complex in the K2O vs. Si2O diagram.The diagram shows the subdivisions of Le Maitre et al. (2004) (broken lines) and ofRickwood (1989) (nomenclature in parentheses). The shaded bands are the fields inwhich the boundary lines of Peccerillo and Taylor (1976) are located.

Fig. 10. Sliding normalization diagram comparing the Fomopéa plutonic complexto the reference syn-shear HKCA of the Yenchichi–Telabit series (Liégeois et al.,1998). The Fomopéa plutonic complex displays a strong potassic trend.

Fig. 9. (a) SiO2 vs. Na2O + K2O diagram, petrological subdivisions from Middlemost (1994); alkaline–subalkaline boundary line from Irvine and Baragar (1971). (b) Plot of themolecular ratio Al2O3/(Na2O + K2O) vs. Al2O3/(CaO + Na2O + K2O) after Maniar and Piccoli (1989).

Fig. 11. Harker diagrams of


syn-shear granitic group from Aïr (Niger), sharing a similar geotec-tonic setting (Liégeois et al., 1998; Fig. 13). In spidergrams (Fig. 13),negative Nb, Ta and Ti anomalies are common and higher in theBmG and EsG than in the BHG. In addition, Th, Ce and P negativeanomalies are common in the EsG, except in K47 for the Ce andin K51 for P which rather shows slight positive anomalies; Sr neg-ative anomalies occur in the BmG; weak P negative anomalies areobserved in the BHG and Zr negative anomalies occurs in dioritesand some BHG samples. HREE enrichment relative to chondrite(Thompson, 1982) is higher in the BHG group than in the others.Globally, Fomopéa shows an enrichment of LILE over HFSE andNb–Ta negative anomalies, both characteristics of high-K calc-alka-line or alkali-calcic series. This is shown with the similar patternsof the Aïr Pan-African syn-shear granitic group (Liégeois et al.,1998; Fig. 13).

selected trace elements.

Fig. 12. Chondrite-normalized REE patterns for the Fomopéa plutonic complex. The normalizing values are from Nakamura (1974). Fomopéa complex rocks are compared toAdma and Iforas granitoids from syn-shear HKCA granitoids from the Tuareg shield (Liégeois et al., 1998).


7. Discussion

7.1. Nature and origin of the Fomopéa magmatism

The petrography, mineral chemistry and whole-rock geochem-istry of the Fomopéa pluton are those of I-type calc-alkaline grani-toids. More precisely, the calc-alkalic BHG rocks are similar toamphibole-rich calc-alkaline granitoids (ACG) and the alkali-calcicBmG and EsG rocks are comparable to K-rich and K-feldspar por-phyritic calc-alkaline granitoids (KCG) of Barbarin (1999). Follow-ing the terminology of Sha and Chappell (1999), the BHG with itsbiotite and hornblende, SiO2 contents between 60 and 69 wt.%,and ASI < 1, correspond to the mafic I-types, whereas the others,because of their higher SiO2 contents and their metaluminous toweakly peraluminous compositions correspond to the felsic I-types.

According to Barbarin (1999), both the ACG and KCG are derivedfrom mixing of mantle-derived basaltic magmas and crustal meltswith varying proportions. The coexistence of mafic enclaves andgneissic country rocks in the Fomopéa plutonic complex indicatesmagma mixing on the one hand and interaction with the countryrocks on the other hand. Th/La ratios suggest interaction betweenmantle and continental sources (Fig. 14).

Assessing the interaction between mantle and continental crustrequires the use of Nd and Sr isotopes, recalculated to the age ofintrusion (620 Ma). In the area of Fomopéa, the continental base-ment is represented by Eburnian (ca. 2 Ga) and Archaean rocks ofthe Congo craton and within the Yaoundé nappes. Part of this base-

ment consists of Rb-depleted granulitic rocks. The Nd–Sr signature(Fig. 15) at 620 Ma of this basement is then in strong contrast withthe mantle and there is no evidence for a juvenile continental crustat depth. The Fomopéa pluton displays a trend from a preponder-antly mantle-derived signature represented by the diorites(87Sr/86Sr): 0.703, eNd = +4, Nd TDM: 0.9 Ga), towards the edenitesyenogranite (EsG) displaying 87Sr/86Sr up to 0.7085, eNd down to�15.8 and a Nd TDM close to 3 Ga. The other samples lie betweenthese two extremes (Fig. 14). This indicates a source combiningan old felsic granulitic crust and a mafic mantle-derived magma(Fig. 15) probably at the origin of the heat needed for the crustalmelting (Bonin, 2004). At 620 Ma, the most depleted mantle has87Sr/86Sr = 0.70219 and eNd = + 8.6. The diorites have a more en-riched signature that can correspond either to such a depletedmantle with crust participation or to a more enriched mantle.The distinction between these two possibilities cannot be resolvedhere but whatever, the mantle signature in these rocks is largelypreponderant. The felsic continental crust must include an Archae-an segment for generating Archaean TDM. A Palaeoproterozoic seg-ment can have also participated as most of the TDM ages arebetween 1 and 2 Ga. The close association of Archaean and Palae-oproterozoic rocks in the area supports such an interaction (Penayeet al., 2004; Tchakounte et al., 2007). This evidence thus links theFomopéa magmatism with the continental crust melting generallyevoked for other granitoids in the West Cameroon part of the Cen-tral Africa Pan-African fold belt, on the basis of their strong nega-tive eNd (e.g. the Ngondo complex, Tagne-Kamga, 2003; theBatoum granitoids, Nzolang et al., 2003 and Nzolang, 2005).

Fig. 13. Chondrite-normalized spidergrams using the normalization factors of Thompson (1982). Fomopéa complex rocks are compared to Adma and Iforas granitoids fromsyn-shear HKCA granitoids from the Tuareg shield (Liégeois et al., 1998).

Fig. 14. Th vs. Th/La diagram from Plank (2005) showing the link between thedifferent groups of Fomopéa rocks. Diorite sample occupies the array of mantlemafic rocks.


7.2. The Fomopéa plutonic complex and the evolution of the CameroonPan-African fold belt

The geochemical and mineralogical characteristics of granitoidsare related to the nature of their protolith, as well as to the geody-

namical environment under which their source magma formedand evolved (Roberts et al., 2000). The post-collisional environ-ment is particularly rich in calc-alkalic and alkali-calcic granitoidsoriginating from the remobilization of an older continental crust(Liégeois et al., 1998). The field relationships of the Fomopéa com-plex define its syn- to late kinematic emplacement relatively to theD2 deformation. It is NE–SW elongated and has an emplacementage between 621 and 613 Ma (U–Pb ages) which corresponds tothe end of crustal thickening in the area. The Fomopéa plutonhas been affected by the right lateral wrench movements as shownby the Fomopéa Rb–Sr isochron reset to ca. 572 Ma. This tectonicevent coincides with the emplacement of most of the post-colli-sional high-K calc-alkaline granitoids in the central domain ofthe Central Africa Pan-African fold belt (Nguiessi-Tchankamet al., 1997; Tagne-Kamga, 2003; Toteu et al., 2004, 2006a).

All these granites are intruded along the Central CameroonShear Zone (CCSZ; Fig. 1) and the parallel Sanaga Fault (SF;Fig. 1). To the south of the latter, the Pan-African Yaoundé nappeswere thrust towards the Archaean craton. The Fomopéa pluton andits associated granitoids are located just to the north of an areacharacterized by a thick lithosphere as indicated by seismic tomog-raphy (Fig. 16). We can then suggest that the CCSZ and the SF markthe northern lithospheric boundary of the Congo craton and thatthe intrusion of these post-collisional granitoids is due to thepost-collisional movements that occurred along this boundary.Such movements could occur during the start of a metacratonicevolution of the cratonic boundary, i.e. its fracturing by shear

Fig. 15. 87Sr/86Sr620Ma vs. eNd620Ma showing the evolution of the rocks from the Fomopéa plutonic complex, compared to the Batoum granitoids (Nzolang et al., 2003) and theNgondo plutonic complex (Tagne-Kamga, 2003), which are mainly generated by crustal reworking. The Fomopéa plutonic complex defines a trend from the mantle array(diorites) towards an Archaean/Palaeoproterozoic continental crust, represented by both a Rb-depleted granulitic lower crust and a Rb-undepleted amphibolite upper crust.

Fig. 16. Horizontal tomographical cross section of Africa showing the shear-wave velocity variation (in %) at 200 km depth (Pasyanos and Nyblade, 2007). This shows that theFomopéa pluton is located along the northern boundary of the thick lithosphere of the Congo craton.



zones, invasion by mantle melts and the partial melting of thecrust, but with the preservation of most of its cratonic rheology(Abdelsalam et al., 2002; Liégeois et al., 2003; Ennih and Liégeois,2008). In such metacratonic areas, linear lithospheric delaminationalong mega-shear zones can occur, favouring the ascent of mantle-derived magmas and consequent crustal melting, giving rise togranitoids (Acef et al., 2003; Liégeois et al., 2003), similar to the Fo-mopéa pluton. The Fomopéa pluton would thus mark the onset ofthis stage. This metacratonization occurred during the convergencebetween the Congo craton and the Saharan metacraton (Abdelsa-lam et al., 2002). Other metacratonic boundaries rich in granitoidsare known on the southern boundary of the Bangweulu craton (DeWaele et al., 2006) and on the northern boundary of the West Afri-can craton (Ennih and Liégeois, 2008).

8. Conclusions

The Fomopéa plutonic complex is located at the south-westernedge of the Central Cameroon mega Shear Zone (CCSZ). It com-prises biotite–hornblende granitoids (BHG) associated with dior-ites and amphibolites, a biotite monzogranite (BmG) and anedenite syenogranite (EsG). The first rock types belong to a calc-alkalic series and the last two to an alkali-calcic series, all beingI-type high-K calc-alkaline and metaluminous to weaklyperaluminous.

The Fomopéa protolith corresponds to the Archaean and Palae-oproterozoic granulitic lower crust most probably that of the Con-go craton melted by mantle-derived magmas, with mixed isotoperatios (Sr620 = 0.7030–0.7085 and eNd620 = �16 to +4 with TDM mod-el ages from 3 Ga to 0.9 Ga). Some relics of dismembered Palaeo-proterozoic crust (2.1 Ga) are observed to the south of theFomopéa area (Kékem granulites; Penaye et al., 2004).

We associate the origin of the Fomopéa pluton to the Pan-Afri-can convergence between the Congo craton and the Saharanmetacraton that initiated the metacratonization of the northernboundary of the Congo craton. Post-collisional transpressive move-ments (strike-slip partitioned; Tagne-Kamga, 2003; Ngako et al.,2003) along the CCSZ (sinistral stage) and the SF induced linearlithospheric delamination allowing the uprise of mantle magmas(Black and Liégeois, 1993), triggering the partial melting of theold lower continental crust of the Congo craton’s northern bound-ary. Mixing of the two types of magmas generated the Fomopéapluton and the other Cameroon post-collisional granitoids. The Fo-mopéa pluton (621–613 Ma) corresponds to the initiation of theprocess (transition between crustal thickening and left lateralwrench movements), the climax being dated at ca. 580 Ma (late leftlateral wrench movements to early right lateral wrench move-ments) during which most of the Cameroon granitoids intruded.

Acknowledgments

This study was performed during the stay of the first author in‘‘Christian-Albrechts Universität Kiel” and ‘‘GeowissenschaftlichesZentrum Göttingen, Universität Göttingen” financed by the Ger-many Academic Exchange Organisation (DAAD). MK is grateful toDietrich Ackermand of ‘‘Mineralogisches Institut, Universität Chris-tian-Albrechts zu Kiel” and Gerhard Wörner of ‘‘Universität Göttin-gen” for their supervision. U- Pb data were carried in the projectIGCP-470 by Sadrack Félix Toteu who is friendly acknowledged.Warm thanks are due to James Roberts and Abderrahmane Soulai-mani for their detailed and thoughtful reviews that improved ni-cely this paper.

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Documents

Nature, origin and significance of the Fomopéa Pan-African high-K calc-alkaline plutonic complex in the Central African fold belt (Cameroon)